Many devices can be produced via semiconductor processing. Semiconductor processing, like any other type of manufacturing, involves moving material through a series of transformative steps until a finished product is produced. Different steps have different complexity and incur different costs. A manufacturer that can produce a product with fewer steps, simpler steps, or less costly steps can often produce the product more inexpensively.
Most semiconductor processing is performed using silicon wafers as a substrate. Some small wafers have a 50 millimeter (mm) diameter and are usually thicker than 200 micrometers (microns). The most common wafers have diameters of 150 mm, 200 mm or 300 mm and are usually thicker than 400 microns.
Silicon wafers are produced from a single crystal. A crystal is a solid material in which the atoms occur in a repeating pattern. The repeating pattern can be used to define a crystal's main crystallographic axes. A flat surface on a crystal is called a face. The relationship between a crystal face and the main crystallographic axes can be expressed in terms of a Miller Index. An example of a Miller Index is (110). One skilled in crystallography can, given knowledge of a crystal and a face's Miller Index, determine exactly how the face intersects the repeating pattern of atoms that forms the crystal. Similarly, a crystal direction refers to a specific direction along the repeating atomic pattern. For example, a (111) direction refers to a specific direction along the main crystallographic axes.
Some semiconductor devices, such as through the wafer (TTW) flow sensors on silicon substrates, have structures requiring costly manufacturing steps. One such structure is a thermal isolator. A thermal isolator can be produced on a silicon substrate by etching a wide deep hole in the substrate and then filling the hole with a thermal insulator. A thermal insulator is a material that has low thermal conductivity. A wide deep hole can be produced fairly inexpensively, but requires an expensive process to fill. For example, achieving good thermal isolation using silicon dioxide, also known as oxide, as the insulating material requires an oxide thickness exceeding 30 microns and a width exceeding 100 microns. Depositing over 30 microns of oxide, however, is an expensive process step.
Another costly structure is a TTW electrical connection. The connection must go completely through the wafer which can be 750 microns thick. Furthermore, the TTW electrical connection must be large enough to carry the amount of electrical current that is required of it. A thick wire can carry more electrical current than a thin wire of the same material. Similarly, a large TTW electrical connection can carry more electrical current than a small one.
Deep reactive ion etching is capable of producing a hole from one side of a wafer to the other large enough for a TTW electrical connection, but is expensive. After the hole is produced, its sidewalls can be oxidized and then it can be filled with an electrically conductive material to produce the TTW electrical connection. Filling the hole with electrically conductive material is also expensive. Current technology requires two expensive steps for producing a TTW electrical connection.
A KOH etch process is capable of producing deep high aspect ratio trenches in (110) oriented silicon. Deep high aspect ratio means that the trench is much deeper than it is wide. The process, however, only produces high sect ratio trenches perpendicular to the (111) direction of the silicon substrate. As such, the KOH etch process has experienced limited use.
Aspects of the embodiments directly address the shortcoming of current technology by exploiting a property of KOH etching of (110) oriented silicon substrates to inexpensively produce deep high aspect ratio trenches. These trenches are then used for inexpensively producing TTW electrical connections and thermal isolators.